How long a paint finish lasts in a greenhouse environment may depend somewhat on how wet your climate is, and your watering habits (how much the steel will be exposed to direct contact with water as you care for plants.)

I'll say right up front that I live in a very dry climate, so I wouldn't hesitate to go with a paint finish on steel. But even if I lived in a wetter climate, I'd probably still be fine with paint. We used to make our living working with handcrafted iron (mild steel) so rust is an old friend/foe that doesn't worry me too much.

I have used a hammered spray paint finish (might well have been Rustoleum) and thought it was a good finish, and if your shop does much exterior iron work, I'd guess their advice was sound. (We did more interior work.)

I'm also a fan of using some kind of a spray primer before applying the final finish. You may want to wipe the surface of the bare steel clean with a rag and some lacquer thinner, before applying the primer coat, just to be sure you get the best primer adhesion. You could then apply two coats of the final paint (two thin coats are usually better than one heavy one) for a really good protected surface.

Over time, if you notice any surface rust forming in any areas, you can always do a bit of preventive maintenance; abrade the surface to remove any loose rust, and reapply more paint.

I tend to view painted steel like any other building material (finished wood, etc) in that you may need to keep an eye on it for a little periodic maintenance, depending on exposure, but I wouldn't be afraid of it.

Sorry, had more coffee, and back with a bit more info, in case you paint your steel. Some steel comes from the mill with a slight oily coating on it; that's why I suggested cleaning the surface with lacquer thinner before priming it.

Let's say your shop used some stock they've had out back on a steel rack, and your frame arrives with a slight orange-y film of early rust...the lacquer thinner should wipe that right off too. And if some of the steel they use has a bit more advanced rust that feels rough, scuff that area with an abrasive pad to remove any loose rust, then wipe with lacquer thinner, then prime and paint.

You don't have to get crazy with surface prep, but steel is kind of like wood...the better bond you get with your paint products, the longer your finish will last. :-)

Keep in mind that the metal may rust out from the inside faster than from the outside. Hot dip galvanizing will take care of the unseen interior. If it does not fit your budget then consider cold galvanizing , a brush on material.

For most paints, it's all about the prep work.... The better the surface is prepared and primed, the longer the paint will last.

Galvanizing is the gold- (or rather zinc-) standard WRT rust prevention. The hot-dipped zinc coating - both inside and out - will oxidize quickly. But it is this thin zinc oxide film that gives it such outstanding rust-preventative properties. It also makes it difficult for most paints to adhere, so it's one or the other.

Now with galvanizing, you don't have to worry about any prep work. The zinc place will dip the pieces in an alkali (to remove grease and 'organics') followed by a pickling in acid just prior to a dunk in the zinc vat. I had the steel wheels on my Land-Rover dipped circa 1976 and after much hard four wheeling, they still look good today.

I've used Hammerite paint quite a bit. (It was 'invented' in the UK by Finnegans and has been copied by others over here.) The stuff hangs on like grim death; toluene is the solvent. Search for it on line, as some hardware store chains now carry it in-store or for pickup after on-line ordering.

I was thinking was Don just said. You're going to have to put screws or bolts in them somewhere, and that is an opening for water to get in to rust it from the inside.